1. Field of the Invention
The present invention relates to a directional coupler and, more particularly, to a directional coupler for use in a mobile communication device or other suitable electronic apparatus.
2. Description of the Related Art
Directional couplers in which two xcex/4 lines are arranged in parallel on a ceramic substrate, and in which both ends of the respective lines (a main line and a subline) are connected to external electrodes, are known. However, as the size of the directional coupler becomes smaller, the pattern formation area of the ceramic substrate must become smaller. As a result, it becomes difficult to form two parallel linear lines in this reduced area. For this reason, mechanisms in which the lines have a meandering shape or a spiral shape and in which the lines are formed within a small pattern formation area have been adopted. In particular, a similar self-inductance value can be obtained with a spiral-shaped line having a shorter line length than with a linear line.
As a construction in which a main line and a subline are combined, there is what is commonly called a xe2x80x9cside-edge-type constructionxe2x80x9d in which, as described above, a main line and a subline are arranged so as to be adjacent to each other on the same plane (the same layer). Alternatively, there is what is commonly called a xe2x80x9cbroadside-type constructionxe2x80x9d in which a main line and a subline are arranged with an insulating layer provided therebetween.
However, as the directional coupler becomes increasingly smaller, the pattern formation area is further reduced. Therefore, it becomes difficult to form a main line and a subline having the necessary self-inductance value within such a small area. In particular, when the subline cannot achieve a sufficient self-inductance value, a problem arises in that the isolation of the directional coupler becomes poor.
Furthermore, even if the line width of a main line and a subline is decreased simply to obtain the necessary self-inductance value, the resistance value of the line is caused to increase, resulting in an increase in the transmission loss of a signal. Since this causes an increase in the power consumption, this is a problem with regard to a mobile communication device, particularly, a battery-driven communication device, which problem cannot be ignored.
In order to overcome the problems described above, preferred embodiments of the present invention provide a small directional coupler in which a main line and a subline have a sufficient self-inductance value and in which insertion loss is very small.
According to a preferred embodiment of the present invention, a directional coupler includes a main line through which a high-frequency signal is transmitted, and a subline, provided on the same plane as the main line, which is electromagnetically coupled to the main line at a portion where the main line and the subline oppose each other, wherein the self-inductance value of the main line is smaller than the self-inductance value of the subline.
Here, as a construction in which the self-inductance value of the main line is lower than the self-inductance value of the subline, for example, the line width of the subline is narrower than that of the main line. More specifically, the line width of the subline is preferably about 50% to about 90% of the line width of the main line.
With the above-described unique construction, for the subline requiring a large self-inductance value, a large self-inductance value is secured by making the line width relatively narrow. In contrast, for the main line which does not require a large self-inductance value in comparison with the subline, the resistance value of the line can be minimized by making the line width relatively wide. At this time, by setting the electrode thickness of the main line to about 5 xcexcm or more and by setting the ratio of the electrode thickness of the main line to that of the subline at about 2:1, the combined resistance value of the main line and the subline is decreased further, and transmission loss of a signal can be reduced.
Furthermore, as a result of multilayering the main line and the subline arranged on the same plane with an insulating layer provided therebetween and electrically connecting the main lines of each layer and the sublines of each layer in series through via holes provided in the insulating layers, respectively, a directional coupler of a multilayered structure can be obtained. For this directional coupler, since the line length of each of the main line and the subline can be lengthened, a higher degree of coupling can be obtained at high-frequency bands, and a sufficient degree of coupling can be obtained also at low-frequency bands.
According to another preferred embodiment of the present invention, a directional coupler includes a main line through which a high-frequency signal is transmitted, and a subline that is multilayered with the main line with an insulating layer provided therebetween, the subline being electromagnetically coupled to the main line along a portion where the main line and subline oppose each other, wherein the line width of the subline is narrower than the line width of the main line, and the self-inductance value of the main line is smaller than the self-inductance value of the subline.
Here, preferably, a grounding electrode opposes at least one of the lines of the main line and the subline with an insulating layer provided therebetween. As a result, a directional coupler of what is commonly called a xe2x80x9cbroadside-type constructionxe2x80x9d is obtained.
According to various preferred embodiments of the present invention, since the main line and the subline are electromagnetically coupled to each other along a portion where the main line and subline oppose each other on the same plane and since the self-inductance value of the main line is lower than the self-inductance value of the subline, a high degree of isolation is obtained, and insertion loss is greatly decreased. In particular, by setting the line width of the subline at about 50% to about 90% of the line width of the main line, a high degree of isolation is achieved also in the main line and the subline provided in a small pattern formation area, and characteristics can be improved without increasing the size of the directional coupler.
Furthermore, in the directional coupler of what is commonly called a xe2x80x9cbroadside-type constructionxe2x80x9d, by setting the line width of the subline to be narrower than the line width of the main line and by decreasing the self-inductance value of the main line to be less than the self-inductance value of the subline, a small directional coupler in which a main line and a subline have a sufficient self-inductance value and insertion loss is small can be obtained.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.